1. Field of the Invention
The invention relates generally to the field of seismic surveying apparatus. More specifically, the invention relates to methods and apparatus for making marine seismic sensor streamer cables.
2. Background Art
Marine seismic surveying is typically performed using “streamers” towed near the surface of a body of water. A streamer is in the most general sense a cable towed by a seismic vessel. The cable has a plurality of seismic sensors disposed at spaced apart locations along its length. The sensors are typically hydrophones, but can also be any type of sensor that is responsive to the pressure in the water, or to changes in the pressure with respect to time. The sensors may also be any type of particle motion sensor or acceleration sensor known in the art. Irrespective of the type of such sensors, the sensors generate an electrical or optical signal that is related to the parameter being measured by the sensors. The electrical or optical signals are typically carried along electrical conductors or optical fibers carried by the streamer to a recording system. The recording system is typically disposed on the seismic vessel, but may be disposed elsewhere.
In a typical marine seismic survey, a seismic energy source is actuated at selected times, and a record, with respect to time, of the signals detected by the one or more sensors is made in the recording system. The recorded signals are later used for interpretation to infer structure of, fluid content of, and composition of rock formations in the Earth's subsurface.
A typical marine seismic streamer can be up to several kilometers in length, and can include hundreds of individual seismic sensors. Because of the weight of all of the materials used in a typical marine seismic sensor, because of the friction (drag) caused by the streamer as it is moved through the water, and because of the need to protect the sensors, the electrical and/or optical conductors and associated equipment from water intrusion, a typical seismic streamer includes certain features. First, the streamer includes one or more strength members to transmit axial force along the length of the streamer. The strength member is operatively coupled to the seismic vessel and thus bears all the loading caused by drag (friction) of the streamer in the water. The streamer also typically includes, as previously explained, electrical and/or optical conductors to carry electrical power and/or signals to the various sensors and to signal conditioning equipment disposed in the streamer in some streamer types. The electrical and/or optical conductors also carry signals from the various sensors to a recording station. The streamer typically includes an exterior jacket that surrounds the other components in the streamer. The jacket is typically made from a strong, flexible, acoustically transparent plastic such as polyurethane, such that water is excluded from the interior of the streamer, and seismic energy can pass essentially unimpeded through the jacket to the sensors inside the jacket. A typical streamer also includes devices placed periodically throughout the streamer, called spacers, that provide mechanical stability and selected physical properties to the streamer. The spacers can be of various types which fix the positions of the “harness” (the assembled sensors and their electrical and/or optical connections and conductors therefor) and the strength members, provide mounting fixtures for electronic devices or the sensors themselves, and provide other functions such as providing the streamer with a selected degree of buoyancy. Some of the spacers, therefore, may be fabricated from material chosen primarily for the specific gravity. The number of and spacing of the various spacers are chosen to provide selected overall buoyancy for the streamer as necessary.
A seismic streamer, including the various components described above, is typically made by assembling the various spacers to the one or more strength members. Some of the spacers have seismic sensors embedded in them or attached to them. The signal output of the various sensors is coupled to the electrical and/or optical cable as described above. The assembled spacers, sensors and cable are then inserted into the jacket, and the interior space or “void” within the jacket is then filled with a suitable filling material. The void filling material provides elastic support to the streamer jacket, provides electrical insulation to any electronic components inside the streamer and can provide some degree of positive buoyancy for ballasting. Filling materials known in the art include gelled hydrocarbon-based oil, hydrocarbon based oil, visco-elastic polymer or other suitable electrically insulating, acoustically transparent materials.
For practical reasons, streamers are typically made as explained above in limited-length segments, typically about 75 meters each. Each streamer segment includes a termination plate at its axial ends. The termination plates include electrical and/or optical connectors that mate with corresponding connectors in the termination plate of another such streamer segment. The termination plate includes features to couple the strength member such that the axial force can be transmitted along the one or more strength members from segment to segment through coupled termination plates. A typical marine seismic streamer, which may be up to several kilometers long as explained above, may thus made from a large number of streamer segments coupled end to end, and ultimately to the seismic vessel.
Seismic streamer-segment manufacturing known in the art is typically performed as follows. A “mechanical harness” is fabricated using one or more strength members having the same length as the intended length of the segment, as described above, and spacers are affixed to the strength member(s) at selected locations along the length thereof. As previously explained, some of the spacers may provide mounting facilities for one or more seismic sensors, and may also as explained above be made from selected materials that give the streamer segment a selected overall density (generally be substantially neutrally buoyant in water). The assembly of spacers and strength members is referred to as a “mechanical harness.” The one or more strength members are attached at each end to a termination plate. An electrical and/or optical harness that includes one or more insulated electrical conductors and/or optical fibers is typically fabricated contemporaneously with the assembly of spacers and strength member(s), and is subsequently inserted into or otherwise coupled to the mechanical harness. Insertion may be performed by locating the seismic sensors within selected components in the mechanical harness (typically the spacers) and threading or engaging a bundle of electrical and/or optical conductors (“wire bundle”) through appropriate openings in the spacers. Connector inserts are then soldered or otherwise attached to the longitudinal ends of the conductors that are used to transit signals between the segments. Then the connector inserts are installed into respective connector housings. The connector housings are attached to the mechanical harnesses at the termination plates, thus completing assembly of both the internal harnesses.
At this point the internal harnesses are ready to be “skinned” and “filled.” Skinning is the insertion of the internal harnesses into the protective jacket, and filling is the injection of the electrically insulating, possibly buoyant and acoustically transparent material into the jacket to provide the streamer with the mechanical properties explained above. The skinning and filling procedures are generally performed in one operation, however, they can be performed in separate operations. In the single operation method, the harnesses are loaded into a tube or other pressure vessel, and the fill material, in liquid state, is also loaded into the tube. The protective jacket is brought up to the end of the tube and pressure applied in the tube to inject the internal harnesses and fill material into the protective jacket.
As can be readily inferred from the above description, making seismic streamers using manufacturing methods and systems known in the art can be difficult and time consuming. Further, it is necessary to provide manufacturing facilities of sufficient floor length to enable assembly of the harnesses and injection thereof into the protective jacket. The typical area of manufacturing plant dedicated to streamer assembly is divided into two general departments. One department typically performs the harness fabrication and testing. The other department typically performs skinning and filling operations. While increasing the length of the streamer would impose a burden on both of the departments, the burden is more severe on the skinning and filling department. The burden is more severe due not only to the additional floor space that would be required (this is also a burden for the harness assembly department) but to the additional capital equipment such as harness loading tubes, pressure tanks and stainless steel tables equipped with drain systems to collect fill material spillage. Additionally, unlike the bare harness, the filled and skinned section is more difficult to handle since it is more resistant to bending and cannot simply be folded over on itself to be worked on as can be done when sensors and electronics are being installed into the harness. The foregoing requirements have effectively limited the practical length of streamer segments that can be made using known manufacturing methods. Furthermore, some types of the material used to fill the streamer are intended to change state, from a liquid to a gel or semi-solid, after insertion of the harnesses. Some compositions of gel can require cure times of up to 14 days, during all of which the streamer segment must be extended straight out in order to maintain proper mechanical properties in the streamer after gel cure.
Increasing the length of manufactured seismic streamer segments is desirable for a number of reasons. First, increasing the manufactured segment length would provide cost and operational benefits. The cost benefit would result from reduction in the number of connector housings and inserts. Both the connector housings and inserts are costly and any increase in section length directly translates into a corresponding reduction in the number of connector housings and inserts and consequently cost. Operationally, the connector inserts are a frequent failure point for the streamer segments, are by nature negatively buoyant and a source for noise. Therefore reduction in the number of inserts in an operational streamer can increase the overall streamer reliability and operational effectiveness. The negative buoyancy of the connector housings and inserts require addition of a large amount of lower density material for ballasting the streamer and impact flexibility in streamer design by practically limiting the amount of heavier-than-water components (wire, sensors, electronics, etc) that can be added to the streamer section.
There continues to be a need to reduce the time, cost and work space needed to manufacture marine longer lengths of seismic streamer sections.